Miniature pumps and valves have been a topic of increasing interest in recent years within the field of chemical analysis, especially in those applications where a variety of functions including pumping, mixing, metering, and species separation are necessary. In particular, there has been interest in integrating miniature pumps and valves with silicon and glass chip-based analysis systems designed to detect and identify trace amounts of chemical or biological material.
To meet these needs efforts have been made to develop and refine micro-scale pumps that rely on the well-known electroosmotic effect, so-called electrokinetic (“EK”) pumps, and related control and valving mechanisms for these devices. The phenomenon of electroosmosis, in which the application of an electric field to an electrolyte in contact with a dielectric surface produces a net force on a fluid and thus a net flow of fluid, has been known since the nineteenth century. The physics and mathematics defining electroosmosis and the associated phenomenon of streaming potential have been extensively explored in “Introduction to Electrochemistry,” by Glasstone, (1942) pp. 521-529 and by Rastogi, (J. Sci. and Industrial Res., v. 28, (1969) p. 284). In like manner, electrophoresis, the movement of charged particles through a stationary medium under the influence of an electric field, has been extensively studied and employed in the separation and purification arts.
The use of electroosmotic flow for fluid transport in packed-bed capillary chromatography was first documented by Pretorius, et. al. (J. Chromatography, v. 99, (1974) pp. 23-30). Although the possibility of using this phenomenon for fluid transport has long been recognized, its application to perform useful mechanical work has been addressed only indirectly. The present embodiment describes an actuator using an EK pump to drive a piston to perform mechanical work.
EK pumps are typically composed of a nanoporous packing or monolith (pore diameters from 10 to 500 nm) and a pair of high-voltage electrodes. For example, silica acquires a negative surface charge composed of deprotonated silanol groups (SiOHSiO−+H+) when an electrolyte with pH>4 is introduced. As illustrated in FIG. 1, a thin electric double-layer (10 nm for water with 1 mM NaCl) is known to develop adjacent to the walls of such devices. Application of an electric field exerts a body force on ions residing within the double layer and results in ion migration in the direction of the electric field gradient which induces viscous “drag” in the bulk fluid. Adding a flow restriction downstream of the porous EK pump monolith will result in an opposing pressure gradient. Hydraulic work, therefore, may be obtained after the fluid exits the porous EK pump monolith. The pressure-driven flow may be used for various applications, such as flow work against a capillary restriction, driving a piston, expanding a bellows, or fluid compression.
Conversely, external pressure-driven flows in these systems will generate electric fields that may be used to perform electrical work.
Many different microfluidic transducers have been implemented by micromachining of silicon and glass substrates. Transducers with pneumatic, thermo-pneumatic, piezoelectric, thermal-electric, shape memory alloy, and a variety of other actuation mechanisms have been realized with this technology. However, only the thermo-pneumatic and shape memory alloy designs have been incorporated in commercially-available products. Unfortunately, transducers utilizing the aforementioned actuation mechanisms are only able to generate modest actuation pressures and are therefore of limited utility.
What is needed is a transducer that can be used for microfluidic systems that can exert larger actuation pressures over longer distances (i.e., more work per stroke) than can be presently developed by conventional (non-explosive) transducers and provides both rapid “on” and “off” actuation.